The present invention relates to a laser emission controller for an optical data recording apparatus for recording data using a laser beam.
Recently, an optical data recording/reproducing apparatus for optically recording and reproducing data using an optical recording medium, or an optical magnetic recording apparatus for magneto-optically recording, reproducing, and erasing data using a magneto-optical recording medium has been developed. In such an apparatus, an intensity of a laser beam for recording is modulated between a low level and a high level in accordance with a level of data to be recorded, i.e., level "0" or level "1". An intensity of a laser beam for reproduction is low and it may be set equal to the low level of the recording mode. An intensity of a laser beam for erasure is high and it may be set equal to the high level of the recording mode. If the intensity of the laser beam required for each mode varies, the intensity must be controlled in accordance with a corresponding mode. However, since a laser source is very sensitive to changes in environmental conditions such as a temperature, a slight change therein can change an intensity of light. In addition, the source itself is deteriorated due to use over a long period of time, and the characteristics of the source change, thereby posing problems such as a decrease in intensity of light.
In order to cope with this, feedback servo control has been performed using a conventional controller shown in FIG. 1. A laser beam radiated from a rear side of laser 1 is detected by detector 2 including a photodetector such as a PIN photodiode. Servo controller 3 compares the detected value with a reference value and controls a drive signal supplied to laser 1 through drive circuit 4 in accordance with the difference. Reference values include low reference voltage V0 for the reproduction mode and high reference voltage V1 for the recording and erasing modes. Modulator 5 for receiving a data signal to be recorded is connected to drive circuit 4. Modulator 5 modulates a level of the drive signal in accordance with the data signal in the recording mode.
The conventional controller feeds back a monitored detection output to drive circuit 4 for all the modes, i.e., recording, reproduction, and erasing modes in the above manner, thereby controlling changes in intensity of a laser beam in a real-time manner. The feedback servo control poses no problem as long as the laser beams for reproduction and erasure are not modulated. However, the following problems are posed when the laser beam is modulated for recording.
When detector 2 detects an intensity of a laser beam by obtaining the mean value thereof, it does not pose a problem if the mean value is calculated using a method of obtaining substantially constant interval mean values, such as Modified-Modified Frequency Modulation (M.sup.2 FM), Eight-to-Fourteen Modulation (EFM). If a method different from the above method is used, such as Return-to-Bias (RB), Return-to-Zero (RZ), the mean value greatly change due to variations in level "0" and level "1" of the data to be recorded, and hence stable feedback control cannot be performed.
In addition, assume that detector 2 detects an intensity of a laser beam by peak holding. If a recording system to be employed is a so-called wide pulse recording system, it does not pose a problem. However, if a recording system to be employed is a so-called short pulse recording system for recording only timings of changes in level (0 or 1) of data to be recorded, such as RZ and RB, the pulse width of the drive signal is very short, e.g., 10 ns through 100 ns. Therefore, it is difficult to realize a high-speed peak holding circuit. In this case, therefore, a stable feedback signal cannot be obtained.
The above disadvantage also applies to laser beam modulators used in light transmission apparatuses.